The field of the present invention is generally related to computed tomography and, particularly, computerized tomography (CT) imaging using a multi-slice detector with selectable slice thickness.
In a conventional CT system used to produce images of at least a region of interest of the human anatomy a complete scan of the patient is comprised of a set of x-ray attenuation measurements which are made at discrete angular orientations of the x-ray source and detector. Each such set of measurements is referred to in the art as a "view" and the results of each such set of measurements is a transmission profile. The set of measurements in each view may be obtained by simultaneously translating the x-ray source and detector across the acquisition field of view. As the devices are translated, a series of x-ray attenuation measurements is made through the patient and the resulting set of data provides a transmission profile at one angular orientation. The angular orientation of each of the x-ray source and detector is then changed (for example, incremented by 1.degree.) and another view is acquired. In an alternative structure for acquiring each transmission profile, the x-ray source produces a fan-shaped beam which defines a plane that passes through the patient and impinges on a generally arcuate array of detectors situated in the plane of the beam. In a conventional detector array, each detector in the array typically produces a separate attenuation signal and the signals from all the detectors are separately acquired to produce the transmission profile for the indicated angular orientation. As in the first structure, the x-ray source and array detector are then revolved to a different angular orientation and the next transmission profile is acquired.
The acquired transmission profiles are then used to reconstruct an image which indicates the x-ray attenuation coefficient of each voxel or volumetric element in the reconstruction field of view. These attenuation coefficients are convened to integers called "CT numbers," which are used to control the brightness of a corresponding pixel on a CRT display. An image which reveals the anatomical structures in a slice taken through the patient and oriented normal to the axis of the gantry aperture is thus produced.
In clinical applications the thickness of the slice taken through the patient may be varied from relatively thin (1 mm) to relatively thick (10 mm). The slice thickness is typically controlled by an adjustable collimation device which is positioned between the patient and the x-ray source. One such collimation device is described in U.S. Pat. No. 4,991,189 issued Feb. 5, 1991, which is owned by the assignee of the present invention.
As the thickness of the slice is increased, the reconstructed image becomes more susceptible to partial volume artifacts. The CT number at each image pixel represents the attenuation of a given x-ray beam portion passing by the corresponding voxel in the patient. For infinitesimally thin beams, an accurate measurement of the line integral attenuation along the x-ray beam could be made, provided sufficient imaging energy can be detected, so that the CT number reflects a true average attenuation introduced by all of the material in the corresponding patient voxel. However, for x-ray beams having a finite thickness, and where the attenuation introduced by the material is substantially inhomogeneous in the thickness direction, an accurate measurement of the average x-ray beam attenuation across the beam thickness direction is not achieved. This inaccuracy is especially pronounced, for example, in voxels which contain a boundary between highly attenuating material such as bone and soft tissues. Because of the nature of the image reconstruction process, this inaccuracy not only affects the corresponding image pixel, but also surrounding pixels. This results in image artifacts which interfere with the diagnosis of soft tissue features.
U.S. Pat. No. 5,241,576, issued Aug. 31, 1993 owned by the assignee of the present invention and which is herein incorporated by reference, describes an x-ray CT scanner which, in conjunction with logarithmic preprocessing techniques, can produce images from one or more slices of attenuation data with reduced partial volume artifacts. It is desirable to provide an improved CT system which eliminates the need for such logarithmic pre-processing techniques and yet is capable of conveniently utilizing a multi-slice detector. It is further desirable to provide a switching technique which allows for selecting slice thickness with a reduced number of electronic circuits and which in turn improves the overall signal-to-noise ratio of the system with the benefit of lower x-ray dose to the patient.